The present invention relates generally to gas turbine engines, and, more specifically, to low NOx combustors therein.
Industrial, power generation gas turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases flow to a turbine that extracts energy therefrom for driving a shaft to power the compressor and producing output power for typically powering an electrical generator for example. The engine is typically operated for extended periods of time at a relatively high base load for powering the generator to produce electrical power to a utility grid for example. Exhaust emissions from the combustion gases are therefore a concern and are subject to mandated limits.
More specifically, industrial gas turbine engines typically include a combustor designed for low exhaust emissions operation, and in particular for low NOx operation. Low NOx combustors are typically in the form of a plurality of burner cans circumferentially adjoining each other around the circumference of the engine, with each burner can having a plurality of premixers joined to the upstream ends thereof. Each premixer typically includes a cylindrical duct in which is coaxially disposed a tubular centerbody extending from the duct inlet to the duct outlet where it joins a larger dome defining the upstream end of the burner can and combustion chamber therein.
A swirler having a plurality of circumferentially spaced apart vanes is disposed at the duct inlet for swirling compressed air received from the engine compressor. Disposed downstream of the swirler are suitable fuel injectors typically in the form of a row of circumferentially spaced-apart fuel spokes, each having a plurality of radially spaced apart fuel injection orifices which conventionally receive fuel, such as gaseous methane, through the centerbody for discharge into the premixer duct upstream of the combustor dome.
The fuel injectors are disposed axially upstream from the combustion chamber so that the fuel and air has sufficient time to mix and pre-vaporize. In this way, the premixed and pre-vaporized fuel and air mixture support cleaner combustion thereof in the combustion chamber for reducing exhaust emissions. The combustion chamber is typically imperforate to maximize the amount of air reaching the premixer and therefore producing lower quantities of NOx emissions. The resulting combustor is thereby able to meet mandated exhaust emission limits.
Lean-premixed low NOx combustors are more susceptible to combustion instability in the combustion chamber as represented by dynamic pressure oscillations of the combustion flame, which if suitably excited can cause undesirably large acoustic noise and accelerated high cycle fatigue damage to the combustor. The flame pressure oscillations can occur at various fundamental or predominant resonant frequencies and higher order harmonics thereof. The flame pressure oscillations propagate upstream from the combustion chamber into each of the premixers and in turn cause the fuel and air mixture generated therein to oscillate or fluctuate.
For example, at a specific flame pressure oscillation frequency, the pressure adjacent to the fuel injection orifices varies between high and low values which in turn causes the fuel being discharged therefrom to vary in flowrate from high to low values so that the resulting fuel and air mixture defines a fluctuating fuel and air concentration wave which then flows downstream into the combustion chamber wherein it is ignited and releases heat during the combustion process. If this heat release from the fuel concentration wave matches in phase the corresponding flame pressure oscillation frequency, excitation thereof will occur causing the pressure magnitude to increase in resonance and create undesirably high acoustic noise and high cycle fatigue damage.
In the parent applications identified above, combustion dynamic stability is enhanced by mis-matching the phase of the heat release from the fuel concentration wave with the phase of the flame pressure oscillation (that is, the high fuel concentration should be 180.degree. out-of-phase with the high pressure oscillation) at one or more specific frequencies to uncouple the cooperation therebetween and attenuate the flame pressure oscillation thereby. The present invention provides further improvements in dynamically uncoupling the fuel from the combustion flame pressure oscillation for reducing combustor instabilities.